The design of a coherent radar frequency transmitter is an important element in a range of modern radars. Coherent radar transmission consisting of either continuous wave or pulsed operation is the basis of doppler radars used primarily against aircraft targets. The signal processing of the radar is able to make use of the coherent nature of the transmitted signal to separate the target return from interfering reflections particularly from ground clutter. Continuous wave and pulse doppler radars are used by aircraft and missile seekers in air to air attack.
In order to provide a coherent signal it is known to provide a low power, spectrally pure source which is amplified through an amplifier chain consisting of between one and very many amplification stages to give the required power output. These stages may consist of wideband amplifiers or injection locked oscillators acting as amplifiers where the characteristics of the pure input signal are imposed on the injection locked oscillator to give a spectrally coherent output signal.
An example of a high gain, single stage amplifier is a Travelling Wave Tube. These devices and their power supplies are particularly large, so for small radar and missile applications a chain of lower gain amplifiers is often preferred. Examples of these lower gain devices are solid state amplifiers (FET and IMPATT) and tubes such as magnetrons and cross field amplifiers.
The critical component in the transmitter chain will be the highest power, output stage and it is particularly important to get this device to operate at maximum efficiency and with the minimum of extra noise added to the input signal. If the output stage is an injection locked oscillator then it is important to ensure that the injection locking signal is not too close to the edge of the injection locking bandwidth of the output stage, otherwise extra power may be needed to injection lock the oscillator and extra noise will be added to the transmitted signal. An example of such a device is a high mean power, injection locked magnetron.
It is found in practical devices that the injection locked output stage is likely to have an injection locked bandwidth that is smaller than the system bandwidth. Moreover drifts in the oscillator frequency with age or temperature may move the centre frequency of the device over quite large ranges often larger than the injection locking bandwidth of the device. This may lead not only to degradations in performance but the complete failure of the transmitter chain if the input signal to the output stage is no longer within its bandwidth.
In order to avoid such problems, extensive development can be undertaken on the higher power output stages to increase the injection locking bandwidth and reduce drifts and uncertainties. However such work is very expensive and variations in frequency will still occur.